1. Field of the Invention
This invention relates to motor control circuits and more particularly to motor control circuits for speed regulation, specifically speed control circuits for DC motors.
2. Description of the Prior Art
The use of back or counter EMF voltage of a motor for monitoring motor speed is known. For example, in U.S. Pat. No. 3,539,893, a circuit samples the back EMF of a motor and uses that signal to vary the amount of energy applied to the motor. Current is applied to the motor for variable lengths of time as a function of the back EMF. The back EMF produces a voltage across a capacitor that controls the period of a single-shot multivibrator and thus controls current flow to the motor.
In U.S. Pat. No. 4,549,122, the full load current for a motor is regulated in response to the counter EMF of the motor. By reducing the current, operation of the motor at higher speeds is affected. However, this circuit relies upon a three-phase AC source and is not particularly relevant to controlling a DC motor.
In U.S. Pat. No. 4,266,168, the back EMF voltage of a DC motor is periodically sampled and compared against a reference set point voltage to produce a signal for controlling an up/down counter. The output of the up/down counter is input to a multiplying digital-to-analog converter having an output which is fed back to a summing junction for developing the DC voltage applied to the motor. However, a system which uses sampling techniques may not be suitable if a pulsed output must be avoided.
Moreover, the known circuitry using back EMF for motor speed control possesses substantial disadvantages, particularly when considered for use in the speed control circuit of a DC motor driven by an analog input. Although the known AC circuitry is incompatible and unsuitable in DC motor control applications, the pulsed input typically used to effect speed corrections in motor control circuits for DC motors may also be unsuitable for certain motor drive systems.
For example, a DC motor driven by a microprocessor controlled analog output can require a smoothed speed correction input which is compatible with its analog input and which avoids the spiked input to the motor of prior circuits to improve circuit efficiency and to increase and maximize brush and gear box life.